Explore Air

Visibility Monitoring


One of the clearest days at
Yosemite National Park, California

Our national parks and wilderness areas offer stunning mountain vistas and scenery full of unique landscapes and geologic features. The enjoyment and appreciation of these are linked to one's ability to see clearly through the atmosphere. Unfortunately, air pollution affects our ability to see clearly. Small particles suspended in the atmosphere, mostly as a result of human-caused air pollution, often create haze -- a grey or white veil over the scene that scrubs it of its colors, forms, and textures and the lessens the visitor's experience of seeing "forever".

One of the haziest days at
Yosemite National Park, California

The National Park Service and the U.S. Environmental Protection Agency (EPA) first began long-term visibility monitoring at selected national parks in 1979. In 1985, a national visibility monitoring program was established called Interagency Monitoring of Protected Visual Environments, or IMPROVE. IMPROVE is a cooperative effort led by a Steering Committee of representatives from the EPA, U.S. Forest Service, National Park Service, Fish and Wildlife Service, Bureau of Land Management, National Oceanic and Atmospheric Administration, and several interstate air quality management organizations.

The goals of the IMPROVE program are to

  • measure current visibility and aerosol conditions in mandatory Class I areas
  • identify chemical species and emission sources responsible for existing human-caused visibility impairment
  • document long-term visibility trends
  • and with the enactment of the regional haze regulations, to provide visibility monitoring representative of all visibility-protected Class I areas.

More information about IMPROVE is available on the IMPROVE web site. The publications, Introduction to Visibility and Air Quality in the National Parks, 2nd edition, also provide good general discussions of visibility and how it is affected by air pollution.

Three types of visibility measurements are undertaken in the IMPROVE network: particle, scene, and optical. The mass and chemical composition of the suspended fine particulate matter (PM-2.5) and the mass of coarse particulate matter (PM-10) is measured by the particle monitor. Every IMPROVE site has such a monitor.

The appearance of a scene viewed through the atmosphere changes under different particle concentrations and lighting conditions. The visual appearance of a scene is qualitatively documented with 35mm color slides, digital images, or video images.

Optical monitoring measures the ability of the atmosphere to scatter and/or absorb light as it passes through it. Optical monitoring instruments used in IMPROVE include transmissometers and nephelometers.


US Visibility Monitoring Site Map
Visibility is monitored at 50 units of the National Park System. Transmissometers, nephelometers, and automatic camera systems are deployed at selected locations. For a larger map with additional information, and a detailed table, please visit our Visibility Monitoring Locations web page.


Particle Sampler at Big Bend National Park, Texas

The IMPROVE particle monitor consists of four independent sampling modules. Three modules collect only fine particles (PM2.5), while the fourth collects both fine and coarse particles (PM10). The IMPROVE monitor measures mass, chemical elements, sulfate, nitrate, organics, and elemental carbon. The samplers run for 24 hours every third day collecting the particulate matter on filters. These filters are retrieved once a week and sent to contracted laboratories for physical and chemical analyses. Particle data are available on the IMPROVE and VIEWS (Visibility Information Exchange Web System) web sites.

35 mm camera located in Tonto National Monument

Scenic conditions are monitored by automatic camera systems that include cameras that take still photographs three times a day, and web cameras that upload new photos to a web site every 15 minutes. Collections of color images taken at many national parks by these camera systems are available on the IMPROVE web site.

Transmissometer at Grand Canyon National Park, Arizona

Optical measurements (pdf, 338KB) are also made at a handful of parks. These measurements include the attenuation of light over a given distance by transmissometers and light scattering in a sampled volume of air by nephelometers.


DataType Details Access Data
Metadata what, where, and when monitors have operated in national parks Monitoring History Database
Particle elements, sulfate, nitrate, organic carbon, and soil concentrations and extinction estimates IMPROVE website

VIEWS website*
Real-time Scenic and Air Quality Conditions current on-line scenic views, ozone, PM2.5, and/or meteorological data Web Cameras
Scenic Conditions photographic archive IMPROVE website/photos
Visibility total light extinction and extinction due to scattering IMPROVE Optical Data

* The VIEWS website is sponsored by the Regional Planning Organizations for the exchange of information relevant to regional haze, and as such, the website includes many different databases on air quality, including IMPROVE data. The website also utilizes many sophisticated data analysis tools.


2005-2007 Average Visual Range in Kilometers

US Average Visual Range Map 2005-2007
Visibility data collected at national parks and wilderness areas by the NPS and other IMPROVE program participants have helped define spatial and temporal variations in visibility throughout the United States. For a larger map and additional information, please visit our Visibility Monitoring Results web page.


NPS Visibility Monitoring Program Manager Bret Schichtel (970) 491-8581
updated on 07/12/2007  I   http://www.nature.nps.gov/air/monitoring/vismon.cfm   I  Email: Webmaster